Certain amphibious vehicles have a suspension system designed to allow the road wheels to be retracted above the vehicle waterline for use of the vehicle on water, particularly so as to reduce the hydrodynamic drag of the hull sufficiently to enable the vehicle to plane on water. Conveniently, said suspension may be retracted and protracted hydraulically. Alternatively, an air or gas suspension system may be used. Hydraulic struts may be used as described in the applicant's co-pending application, published as WO 01/74612. The mounting of these struts to the vehicle structure may be as described in the applicant's co-pending application, published as WO 02/44006. However, for the purpose of retraction when using a retraction cylinder as a suspension strut, as is shown in our International Patent Application Publication No WO 01/74612, where the invention relates to particulars of such a strut, the operation is only broadly described. A problem with the strut shown in Publication No WO 01/74612 is to ensure that the road wheels of the vehicle are safely lowered from the retracted position.
Since the fluid in the hydraulic suspension struts is substantially incompressible, accumulation space is provided in the arrangement of our co-pending application Wo 01/74612 to allow compression of the struts, so as to give a wheel travel in bump and rebound comparable to a wheel suspension using conventional steel springs. The problem with this arrangement is to control the accumulation, that is, the springing, so that when beaching, where an amphibious vehicle hits an obstacle on one side, the suspension does not cause the vehicle to tilt uncomfortably.
A further problem arises where ride height sensors are provided, and used in a ride height control system. When the vehicle is fording shallow water; passing over crests or hump back bridges; or being driven on water in displacement mode; all of the road wheels will tend to droop fully. The ride height control system will then act to pull the wheels up into the wheel arches. In fords, this may result in the vehicle floating, and thus deviating from its intended course. After crests, this could result in grounding. When driving on water in displacement mode, the keel could ground on leaving water. The cresting scenario lasts for typically less than a second, so the suspension may not have time to adjust, but it is preferred to switch off ride height control whenever the vehicle is on water.
A yet further problem may arise when the vehicle is on water with the wheels raised. Should a particular wheel droop; particularly a rear wheel; and particularly if the vehicle is planing; drag over water could be significantly increased, particularly when the vehicle is cornering. In theory, the use of a strut locking peg would prevent any wheel from drooping on water. However, such locking pegs are usually positioned so as to prevent the vehicle keel from grounding on land when the wheels are protracted. The geometry of the suspension may be such as to prevent a peg positioned for use on land from keeping a raised wheel above the water level in marine mode, particularly when the vehicle is cornering.